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AD8310 log amp frequency response problem

Question asked by Spacerowa on May 3, 2017
Latest reply on May 30, 2017 by jdobler


My AD8310 log amp circuit only differs from the basic connection shown in the data sheet by having 200pF connected to OFLT pin, 680nF input coupling, and I have connected the dc Vout pin4 directly to Vin+ of a 22bit ADC ( not ADI ).

My circuit is built on double sided pcb, ground plane both sides, smt components and recommended decoupling; however no metal box is used for shielding/screening.

I use input matching for measuring power in a 50Ohm circuit with the recommended parallel 52R3 across the input; good for LF, HF frequencies. But for higher frequencies, VHF and up, I am not using any high pass input matching network .

I have calibrated my circuit using a 2-point calibration at 4MHz using 0dBm and -20dBm inputs; slope 24.57mV/dB, intercept -95.37dBm

When I made measurements with inputs set to -10dBm and -20dBm in the frequency range 10MHz-450MHz the output signal reduced by up to 10dB; please see attached graph.

This was completely unexpected and I was very surprised, as the data sheet contains the following statements viz, '-3dB bandwidth 900MHz', 'logarithmic linearity typically within +/- 0.4dB up to 100MHz' and '+/- 0.4dB linearity DC-400MHz'

So what is going on ? Why is the response not flatter ?

Has anyone got round this problem and have any suggestions ?Measured frequency response of AD8310 circuit#